Presently, NOx, SPM (Suspended Particulate Matter), and photochemical oxidant cause air pollution, and the influence on the environment is regarded as a problem. For example, the photochemical oxidant mainly containing a strongly oxidizing substance such as ozone is produced by a photochemical reaction of a pollutant such as NOx or hydrocarbon exhausted from factories, offices, and automobiles when the pollutant is irradiated with the sunlight, and causes photochemical smog.
The photochemical oxidant is almost made of ozone, and measurement stations in various places measure the concentration of ozone by an automatic measurement method such as an ultraviolet absorption method. Ozone gas concentration measurement performed by this automatic measurement method can measure a slight amount of a gas, i.e., a few ppb of a gas, but the measurement is expensive and requires constant setup in order to maintain the accuracy. Also, in automatic measurements performed by these apparatuses, electric power is always necessary, and maintenance and management are essential, so enormous costs are required. In addition, temperature-controlled installation environments and standard gases for calibration must be secured for these measurements.
To accurately investigate the gas concentration distribution in an environment and evaluate the influence on a local environment, it is necessary to increase the number of observation points and perform measurement and investigation on a nationwide scale. However, it is very difficult to perform the above-mentioned automatic measurement method in many observation points. Therefore, demands have arisen for a readily usable, compact, inexpensive ozone gas analyzer and a simple measurement method.
Recently, ozone is attracting attention because it has strong sterilizing power (oxidizing power), and changes into oxygen and produces no harmful substance after decomposition. This is extending the use of ozone to various industrial fields such as water processing, food sterilization, and paper bleaching. Accordingly, reference values of 100 ppb and 8 hrs are set for the ozone concentration as labor environmental standards. In a factory using ozone, it is necessary not only to install ozone alarms, but also to manage the state in which each worker works within the range of the labor standards. This requires a measurement device that can be carried by a worker.
Under these circumstances, ozone gas measurement techniques have been presently extensively developed. Examples are a semiconductor gas sensor, solid-state electrolyte gas sensor, electrochemical gas sensor, and quartz oscillating gas sensor. However, these sensors have been developed to evaluate short-time responses, and only a few sensors have been developed for monitoring requiring measurement data accumulation. When it is necessary to accumulate measurement data, therefore, the sensors must always be operated. Also, a sensing unit of, e.g., the semiconductor sensor must be held at a few hundred ° C., so a large amount of electric power is always necessary to constantly operate the sensor.
Furthermore, the above-mentioned sensors have a sensitivity of about sub-ppm, and hence cannot measure concentrations in real environments, e.g., cannot measure 10 ppb of ozone. Although some semiconductor sensors react to 10 ppb of ozone, the sensor output is nonlinear with respect to the concentration, and the output value largely changes from one sensor to another. This makes comparison difficult when using different sensors. Also, the influence of another gas cannot be neglected in many cases.
There is also a method using a detector tube type gas measurement device. Unfortunately, this method has also been developed to locally measure a very-short-time concentration in a measurement point. This makes it difficult to obtain long-term cumulative measurement data. In addition, this method using a detector tube requires the presence of an operator in a measurement site, and also poses the measurement accuracy problem that different operators read color changes between measurements differently.
On the other hand, ozone detecting paper carrying starch and potassium iodide has been proposed as a simple, high-sensitivity ozone analyzing technique (see reference 1: Japanese Patent No. 3257622). However, this technique disclosed in reference 1 requires a pump for forcedly drawing a gas to be detected, a light source for measurement, and electric power for driving a detector including the pump and light source. Also, a special sheet-like carrier is necessary and must be replaced whenever measurement is performed. This makes cumulative measurement difficult. In addition, the measurement using the detecting paper described above detects all photochemical oxidants instead of ozone.
As another simple, high-sensitivity ozone gas analyzing method, a technique using ozone detecting paper carrying indigo carmine has been proposed (see reference 2: Anna C. Franklin, et al., “Ozone Measurement in South Carolina Using Passive Sampler”, Journal of the Air & Waste Measurement Association, Vol. 54, pp. 1312-1320, 2004). A technique by which a membrane filter is placed on the surface of an ozone detecting sheet carrying a blue indigo dye, and the sensitivity is controlled by adjusting the thickness of the membrane filter has also been proposed (see reference 3: “Operating Instructions for Ozone Monitor”, Part #380010-10, http://www.kandmenvironmental.com/PDFs/ozone.pdf).